Gaussian-like Beam Research Articles

Quasi-Optical Mode Converter for a 0.42-THz TE26Mode Pulsed Gyrotron Oscillator

A quasi-optical (QO) mode converter for a 0.42-THz, TE26 mode, pulsed gyrotron is designed, fabricated, and tested. Based on the geometric optics principle, vector diffraction theory, and the iterative Stratton–Chu method, a computer code is developed to design and optimize the QO mode converter. The scalar Gaussian mode content is about 93.7%, which agrees well with that obtained by the electromagnetic simulation software. The preliminary experiment of the 0.42-THz, TE26 mode QO mode converter shows that a well-focused Gaussian-like beam with 90.4% scalar Gaussian mode content has been observed on the output window by using an infrared video camera. The experimental results agree well with the theoretical prediction.

Intensity properties of flat-topped vortex hollow beams propagating in atmospheric turbulence

The analytical propagation expressions for the average intensity of flat-topped vortex hollow beams in atmospheric turbulence are obtained by using the extended Huygens-Fresnel diffraction integral. And the influences of atmospheric turbulence on the average intensity of flat-topped vortex hollow beams are given by numerical examples. The results show that the flat-topped vortex hollow beams propagating in atmospheric turbulence will evolve into Gaussian-like beam with the propagation distance increasing, and the beam propagating in atmospheric turbulence with larger structure constant Cn2 will lose its initial dark hollow centre more rapidly.

All-fiber laser with intracavity acousto-optic dynamic mode converter for efficient generation of radially polarized cylindrical vector beams

We report an all-fiber laser that emits a radially polarized cylindrical vector beam (CVB) at 1 μm based on an intracavity acousto-optic mode converter. We efficiently generate the CVB, taking advantage of the acousto-optic coupling from the HE11 mode to the TM01 mode in a two-mode fiber. The laser can be switched from emitting a Gaussian-like beam to a radially polarized CVB. Radially polarized CVBs with modal and polarization purities >98% and a maximum power of 65 mW were generated.

Direct Femtosecond Laser Surface Structuring with Optical Vortex Beams Generated by a q-plate

Creation of patterns and structures on surfaces at the micro- and nano-scale is a field of growing interest. Direct femtosecond laser surface structuring with a Gaussian-like beam intensity profile has already distinguished itself as a versatile method to fabricate surface structures on metals and semiconductors. Here we present an approach for direct femtosecond laser surface structuring based on optical vortex beams with different spatial distributions of the state of polarization, which are easily generated by means of a q-plate. The different states of an optical vortex beam carrying an orbital angular momentum ℓ = ±1 are used to demonstrate the fabrication of various regular surface patterns on silicon. The spatial features of the regular rippled and grooved surface structures are correlated with the state of polarization of the optical vortex beam. Moreover, scattered surface wave theory approach is used to rationalize the dependence of the surface structures on the local state of the laser beam characteristics (polarization and fluence). The present approach can be further extended to fabricate even more complex and unconventional surface structures by exploiting the possibilities offered by femtosecond optical vector fields.

Beam self-cleanup by use of self-written waveguide generated by photopolymerization.

A novel method for multimode fiber (MMF) laser-beam cleanup is introduced based on the optically induced growth and interaction of self-written waveguides (SWWs) in a photopolymer material. Theoretically, it is predicted that when the light is introduced into a free-radical photopolymerizable system from a MMF, the incident multichannel and structured beam shape can be caused to merge to form a single-channel Gaussian-like beam under specific exposure and material conditions. Experimental validation was carried out using a dry acrylamide/polyvinyl alcohol (AA/PVA) photopolymer sample. This work opens the door to studies involving self-developing laser beam cleanup and also to possible applications in photonic telecommunication systems and integrated optical devices.

Development of the Multifrequency Gyrotron FU CW GV with Gaussian Beam Output

Gyrotron FU CW GV has been developed as a multifrequency gyrotron for operation over the frequency range from 162 to 265 GHz at frequencies separated by steps of approximately 10 GHz. The oscillation modes were selected; the radii of the caustic surfaces for the electromagnetic waves of the modes had similar values in the waveguide, and it was therefore expected that these modes would be converted into Gaussian beams by a mode converter. In reality, more than ten modes oscillated and the Gaussian-like beams were radiated. A double-disk window with variable spacing maintains the transmittance through the window at a high level over a wide range of frequencies. Using this window, output powers of more than 1 kW were observed for almost all the expected modes.

Highly directional emission from a quantum emitter embedded in a hemispherical cavity.

We report the design of a solid-state, micron-sized hemispherical cavity that yields significantly enhanced extraction efficiency with modest Purcell enhancement from embedded quantum emitters. A simple analytical model provides a guideline for the design and optimization of the structure, while finite-difference time-domain simulations are used for full analysis of the optimum structure. Cavity modes with up to 90% extraction efficiency, a Purcell enhancement factor >2, and a quality factor of ≈50 are achieved. In addition, Gaussian-like far-field beam profiles with low divergence are exhibited for several modes. These monolithic cavities are promising for solid-state emitters buried in a high dielectric environment, such as self-assembled quantum dots and optically active defects in diamond.

Theoretical and Experimental Investigations on the Quasi-Optical Mode Converter for a Pulsed Terahertz Gyrotron

Theoretical and experimental investigations on the quasi-optical (QO) mode converter for a 0.42 THz, TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">26</sub> mode pulsed gyrotron have been carried out. The vector diffraction theory and the multiple iterative algorithms are used to design and optimize the QO mode converter, and the theoretical value for the conversion efficiency is ~88%. The actual conversion efficiency is ~86% by measuring the input and output power of the QO mode converter indirectly with a calorimeter. The field distribution on the output window of the QO mode converter is measured with an infrared video camera, and a well-focused Gaussian-like beam has been observed on the output window. The experimental results agree well with the theoretical predictions.

Improving feature size uniformity from interference lithography systems with non-uniform intensity profiles

The non-uniform intensity profile of Gaussian-like laser beams used in interference lithography (IL) leads to a non-uniform dose and feature size distribution across the sample. Previously described methods to improve dose uniformity are reviewed. However, here we examine the behavior of the non-uniformity from the viewpoint of photoresist response rather than the IL system configuration. Samples with a fixed intra-sample dose profile were exposed with an increasing average dose. A line/space pattern with a period of 240 nm across an area of 2 × 2 cm2 was produced using IL on identical samples using a HeCd laser operated at 325 nm and a Lloyd’s mirror IL system. A binary model of photoresist response predicts that the absolute range of line widths in nanometers should be significantly reduced as the overall sample dose is increased. We have experimentally verified a reduction in the range of line widths within a given sample from 50 to 16 nm as the overall dose is increased by only 60%. This resulted in a drop in the narrowest line width from 120 to 65 nm. An etch process is demonstrated to increase the line width by generating a wider secondary chrome hard mask from the narrowly patterned primary chrome hard mask. The subsequent fabrication of a silicon nanoimprint mold is used as a demonstration of the technique.

Orbital angular momentum correlations with a phase-flipped Gaussian mode pump beam

We report orbital angular momentum (OAM) and angle correlations between signal and idler photons observed when the nonlinear crystal used in spontaneous parametric down-conversion is illuminated by a non-fundamental Gaussian pump beam. We introduce a π-phase step to the transverse profile of the pump, before it impinges on the crystal to create a phase-flipped Gaussian mode, which is a close approximation to an HG10 Hermite–Gaussian-like beam. The correlations in OAM and angular position are then measured holographically using two separate spatial light modulators in the signal and idler arms. We show the transfer of the OAM spectrum of the pump to the down-converted fields, manifested as a redistribution in the OAM correlations consistent with OAM conservation. This corresponds to a modulation of the angular position correlations consistent with the Fourier relationship between the OAM and angle.

Evolution of decentred laser beams propagating through atmospheric turbulence

The changes of the average intensity, the centre of beam gravity and the position of intensity maximum of decentred laser beams propagating through atmospheric turbulence are examined in detail. It is shown that the decentred intensity distribution is amended gradually with increasing the propagation distance and the strength of turbulence, and it becomes an off-axis Gaussian-like beam when the propagation distance and the strength of turbulence become large enough. The centre of beam gravity is independent of both the propagation distance and the strength of turbulence. On the other hand, there are two intensity maxima, and their positions are symmetrical around the propagation z-axis when the propagation distance z is small. With increasing z, there is only one intensity maximum. As z further increases, position of the intensity maximum is further shifted towards the z-axis. When z is large enough, the position of the intensity maximum is unchanged. The unchanged position of the intensity maximum moves further away from the z-axis with an increase in the refraction index structure constant, the decentred parameter and the waist width.

Highly efficient room-temperature Yb:YAG ceramic laser and regenerative amplifier

A room-temperature, diode-pumped, pulsed Yb:YAG ceramic free-running laser with a 78% slope efficiency has been demonstrated. It has been shown that fine tuning of the mode diameter to the diameter of the pumped volume with a ratio of the pump to the mode diameter of 0.6 is important for achieving maximum laser efficiency with a Gaussian-like beam profile. A regenerative Yb:YAG ceramic amplifier with a maximum output energy of 14.5 mJ and a super-Gaussian beam profile is demonstrated.

Quasi-Optical High Purity HE&lt;sub&gt;11&lt;/sub&gt;-Mode Exciter for Oversized Corrugated Waveguide Transmission

Oversized corrugated waveguide transmissions are used to transmit high power millimeter wave for electron cyclotron heating in ITER. The HE11 mode purity of the waveguide components is a critical issue not only in the high power operation but also in the low power operation to evaluate the components. Conventional single Gaussian beam system causes edge diffraction and excites higher order modes at the waveguide aperture. A proposed HE11-mode exciter uses beam interference between two Gaussian beams interference and serves the high purity HE11 mode without the edge diffraction. The HE11-mode exciter consists of a scalar feed horn antenna, quasi-optical mirrors and a beam splitter (combiner). Phase matched mirrors correct the Gaussian-like beam excited from the horn antenna to pure Gaussian beam. Authors report the experimental study of the exciter.

High power millimeter wave experiment of ITER relevant electron cyclotron heating and current drive system

High power, long pulse millimeter (mm) wave experiments of the RF test stand (RFTS) of Japan Atomic Energy Agency (JAEA) were performed. The system consists of a 1 MW/170 GHz gyrotron, a long and short distance transmission line (TL), and an equatorial launcher (EL) mock-up. The RFTS has an ITER-relevant configuration, i.e., consisted by a 1 MW-170 GHz gyrotron, a mm wave TL, and an EL mock-up. The TL is composed of a matching optics unit, evacuated circular corrugated waveguides, 6-miter bends, an in-line waveguide switch, and an isolation valve. The EL-mock-up is fabricated according to the current design of the ITER launcher. The Gaussian-like beam radiation with the steering capability of 20°-40° from the EL mock-up was also successfully proved. The high power, long pulse power transmission test was conducted with the metallic load replaced by the EL mock-up, and the transmission of 1 MW/800 s and 0.5 MW/1000 s was successfully demonstrated with no arcing and no damages. The transmission efficiency of the TL was 96%. The results prove the feasibility of the ITER electron cyclotron heating and current drive system.

Mobility of high-power solitons in saturable nonlinear photonic lattices

We theoretically study the properties of one-dimensional nonlinear saturable photonic lattices exhibiting multiple mobility windows for stationary solutions. The effective energy barrier decreases to a minimum in those power regions where a new intermediate stationary solution appears. As an application, we investigate the dynamics of high-power Gaussian-like beams finding several regions where the light transport is enhanced.

The comparison of the elements for the homogenizing charged particle irradiation

AbstractParticle beams with uniform and well-confined intensity distributions are desirable in some high power beam applications to prolong the target lifetime or to improve the beam utilization. Three kinds of elements had been proposed for the beam homogenizing, such as octupole, pole-piece magnet, and step-like nonlinear magnets. In this paper, the new type of elements called heteromorphic quadrupole and focus sextupole are proposed. The Gaussian-like multiparticle beam redistribution by the octupole, heteromophic quadrupole, step-like nonlinear magnets, and focus sextupole has been simulated by the POISSON and LEADS code. The best redistribution result is obtained by the focus sextupole, and one of the solutions of redistributing beam with big halo can be that of using the focus sextupole and the heteromorphic quadrupole.

πShaper – Refractive Beam Shaping Optics for Advanced Laser Technologies

Laser beam shaping brings to various industrial and scientific laser techniques effects that improve their performance comparing to what can be achieved with using Gaussian or Gaussian-like laser beams: more stability, less tough positioning tolerances make the technologies easier to use, higher efficiency of using of costly laser energy, etc. The task of such a transformation is solved by series of refractive beam shaping optics of field mapping type. This solution is important in irradiating the cathode of Free Electron Lasers, confocal microscopy, biomedical fluorescence techniques, many industrial technologies like welding, cladding, hardening, various laser techniques in photovoltaics, homogenizing of pump radiation by building powerful femtosecond lasers, etc. The refractive beam shapers can be used with TEM00 and multimode laser beams, achromatic design provides the same conditions of beam shaping for several lasers of a certain spectrum range simultaneously, low inherent losses allow to use them with powerful laser sources, particular models can be implemented as Galilean Telescope without internal focusing or as Collimators. This paper will describe the principles of operation, design features of the achromatic refractive beam shapers; there will be presented examples of beam intensity transformation and effects on material processing achieved in several industrial applications.

Gaussian-Like Ion Beam Shaping by Double Scattering

Gaussian-Like Ion Beam Shaping by Double Scattering

Spectral characterization of porous dielectric subwavelength THz fibers fabricated using a microstructured molding technique

We report two novel fabrication techniques, as well as THz spectral transmission and propagation loss measurements of subwavelength plastic wires with highly porous (up to 86%) and non-porous transverse geometries. The two fabrication techniques we describe are based on the microstructured molding approach. In one technique the mold is made completely from silica by stacking and fusing silica capillaries to the bottom of a silica ampoule. The melted material is then poured into the silica mold to cast the microstructured preform. Another approach uses a microstructured mold made of a sacrificial plastic which is co-drawn with a cast preform. Material from the sacrificial mold is then dissolved after fi ber drawing. We also describe a novel THz-TDS setup with an easily adjustable optical path length, designed to perform cutback measurements using THz fibers of up to 50 cm in length. We fi nd that while both porous and non-porous subwavelength fibers of the same outside diameter have low propagation losses (alpha <or= 0.02 cm(-1)), the porous fi bers exhibit a much wider spectral transmission window and enable transmission at higher frequencies compared to the non-porous fibers. We then show that the typical bell-shaped transmission spectra of the subwavelengths fibers can be very well explained by the onset of material absorption loss at higher frequencies, due to strong confinement of the modal fields in the material region of the fi ber, as well as strong coupling loss at lower frequencies, due to mismatch between the modal field diameter and the size of the gaussian-like beam of a THz source.

Experimental Study of a Relativistic Resonant Traveling-Wave Tube With Selective Feedback Provided by Bragg Reflectors

The realization of single-mode generation in a microwave oscillator is considered for a relativistic resonant traveling-wave tube (RTWT) with spiral Bragg reflectors at the ends of a multimode electrodynamic system. An oversized azimuthally symmetric slow-wave structure was chosen so that only two field structures are synchronous with a 1-MeV electron beam. Experiments using different means that allow us to separate these field structures are described. As a result, two radiation patterns were observed separately, which are a narrow Gaussian-like wave beam with a power of up to 1.5 GW and an efficiency of about 20% in 40-ns pulses and radiation corresponding to the E01 mode with four times weaker power. The dependences of RTWT operation on the parameters of the electron beam, the length of interaction space, reflection coefficients, and the amplitude of guide axial magnetic field are investigated in detail.